The present invention relates to an apparatus and a method for ozone-treating biosludges, such as activated sludge, digested sludge and the like, occurring in the biological treatment of aqueous organic wastes and to a method for such ozone treatment.
In an aerobic or anaerobic biological treatment of an aqueous organic waste, biosludges are formed, such as activated sludge and digested sludge. While excessive biosludges formed in the biological treatment system should be removed therefrom, they exhibit a high biological stability, which makes the disposal of such biosludges through biodegradation or biolysis difficult. In addition, they are highly resistent to dehumidification, which makes disposal of such biosludges by dewatering difficult. Under the contemplation of solving the problem of disposal of such biosludges occuring upon biological treatment of aqueous organic wastes, a technique was proposed for a biological treatment of such biosludges, which comprises subjecting the biosludge to an ozone treatment and recycling the so-ozonized biosludge to the original biological treatment of the waste by an activated sludge in order to subject it to a biolysis (Japanese Patent Kokai Hei-7-206088). In this technique, by an ozone treatment, the biosludges are converted into BOD components susceptible to biological metabolization by the living microorganisms in the activated sludge and decomposition into gases and inorganic substances.
The method for ozone-treating excess amounts of the biosludge of this prior art uses an apparatus in which an ozone-containing gas is blown into a layer of a biosludge contained in an ozonizing reactor to cause a gas/liquid contact to facilitate the ozonization of the biosludge. The reactor used in the apparatus of the prior art is designed to be filled with the biosludge up to a liquid depth of about 1 meter or so and to blow the ozone-containing gas thereinto under atmospheric pressure, so that only a relatively low ozone concentration in the liquor can be attained for effecting the ozonization, resulting in a lower yield of ozonization. Thus, it is necessary for increasing the ozonization yield in this prior art to increase the biosludge residence time in the reactor and an efficient ozonization cannot be expected.
For improving such circumstances, a technique was proposed, in which the ozone treatment of biosludge is effected with an ozone-containing gas under an elevated pressure (Japanese Patent Kokai Hei 8-192196). While this technique attains an increased ozone absorption rate due to the elevated pressure, problems still remained, for example, in the poor contacting efficiency of the ozone-containing gas with the biosludge when the gas is simply blown thereinto and, when a gas distribution device, such as a porous gas diffuser disc, is employed for blowing the gas thereinto, it is easily clogged by the biosludge particles deposited thereon.
In order to remedy such problems, a technique has been proposed in which the ozone-containing gas is introduced into the reactor by supplying the gas to a downflow pipe disposed in the reactor for flowing the biosludge downwards therethrough using a circulation pump to cause circulation of the liquor in the reactor together with the gas bubbles supplied. This technique can serve for a stable operation of the apparatus without suffering from the occurrence of clogging of the gas supply element, nevertheless, it tends to suffer from a lower efficiency of the circulation pump and tends to prevent the occurrence of complete mixing. When the original liquor is sprayed over a foam layer for destroying the foam, the spray nozzle often becomes clogged. When a technical measure of distributing the ozone-containing gas by installing a stirrer in the reactor is employed, an increase in the residence time of the biosludge in the reactor is unavoidable, resulting in an increase in a tendency of the occurrence of foaming, together with possible short circuiting or insufficient mixing of the biosludge, causing a decrease in the ozone treating efficiency and reducing the rate of elimination of the excess biosludge.
An object of the present invention is to provide an apparatus and a method for ozone-treating biosludges in which an efficient, absorption of ozone by a biosludge can be attained at a high absorption rate and under a high load to effect an efficient ozone treatment of the biosludge at a high efficiency of power consumption and at a high rate of reduction of the biosludge, while suppressing the occurrence of foaming and preventing clogging of the gas distributor.
The above object of the present invention can be attained by an apparatus and a method as defined below:
(1) An apparatus for ozone-treating biosludges, comprising
a plurality of reactors for effecting the ozone treatment of the biosludge by contacting the biosludge with an ozone-containing gas,
biosludge passages for flowing the biosludge to be ozone-treated through all the reactors in series,
gas supply means for supplying the ozone-containing gas to each of the reactors,
a pressurization means for maintaining each of the reactors under a pressurized condition and
stirrers disposed in each reactor for realizing a nearly equalized ozone absorption rate (g-ozone/hr per reactor) for all the reactors.
(2) A method for ozone-treating biosludges, comprising
passing the biosludge to be ozone-treated through a plurality of ozone-treating reactors in series, while supplying an ozone-containing gas to each of the reactors with the agitation of the biosludge under a pressurized condition in such a manner that a nearly equalized ozone absorption rate is realized in all the reactors.